In recent years, soot discharged from engine, such as a diesel engine or the like, has become a problem. A purification device including a catalyst made of platinum alumina or the like generally intervenes in an exhaust pipe of the engine to remove soot from an exhaust gas. The purification device accommodates a ceramic honeycomb structure for supporting the catalyst material in a container. The exhaust gas containing soot is allowed to pass through the container, which can remove soot from the exhaust gas.
In general, the ceramic honeycomb structure is recycled in the purification device. That is, soot is accumulated in the honeycomb structure used for purification of the exhaust gas. In a recycling process, excessive fuel is burned to increase the temperature of the honeycomb structure, whereby soot accumulated in the honeycomb structure can be burned and removed. The honeycomb structure supporting the conventional catalyst material made of platinum alumina, however, has to be heated at a high temperature of 600° C. or more so as to burn and remove soot. In the recycling process involving such burning and removing steps, much fuel is wasted so as to heat the honeycomb structure at the high temperature, disadvantageously leading to reduction in fuel efficiency.
Thus, a catalyst material to be supported on a honeycomb structure for decreasing a combustion temperature in recycling is required to be developed. Specifically, for example, an alkali-based catalyst material mainly containing an alkali element has been proposed (see Patent Document 1). The honeycomb structure supporting such a catalyst material can burn soot at a relatively low temperature and be recycled. A silver oxide is known to serve as material having a low temperature activity (see, for example, Non-Patent Document 1).
A catalyst to be used for purifying the exhaust gas is proposed to be oxides composed of a crystal structure with mixed layers into which delafossite-type oxides of different crystal types are mixed (see Patent Document 2, for example). When the oxide is used for burning soot, the oxygen stored by the catalyst between the layers serves to constantly maintain an oxygen concentration, but does not have activity of burning soot at a low temperature.
Patent Document 3 describes a catalyst for promoting combustion of particulates from the diesel engine. Specifically, the catalyst described for the purpose of excellent high-temperature thermal resistance is mainly composed of BaAl12O19 in which a part or all of a Ba site is substituted by Ag, and a part of an Al site is substituted by Cr or the like.
Even when all parts of Ba are substituted by Ag, the amount of Ag contained in the catalyst is very small. This is apparent from a chemical formula of BaAl12O19. Such an amount of Ag gives the excellent high thermal resistance, but makes it difficult to burn carbon at a low temperature.
Patent Document 4 describes a delafossite-type composite metal oxide which serves well as an oxidation catalyst. However, specifically, only the oxidation catalyst composed of Ag at the A site, and Cr, Fe, and Co at the B site is described.    Patent Document 1: JP-A-2001-271634    Patent Document 2: JP-A-2000-25548    Patent Document 3: JP-A-1990-261511    Patent Document 4: Patent Document No. 1799698    Non-Patent Document 1: John P. A. Neeft et al., FUEL 77, No. 3, pp. 111-119, 1998